The present invention relates to a system for accurately positioning a workpiece with respect to an array of spaced-apart members. The inventon has particular application to the maintenance of a nuclear steam generator, and specifically to the accurate positioning of a workpiece, such as a fluid lance nozzle, with respect to the heat-exchange tubes of the steam generator.
A typical nuclear steam generator comprises a vertically oriented shell and a plurality of inverted U-shaped tubes disposed in the shell so as to form a tube bundle. Each tube has a pair of elongated vertical portions interconnected at the upper end by a curved bight portion, so that the vertical portions of each tube straddle a center lane or passage through the tube bundle. The tubes are dimensioned and arranged so that, on each side of the center tube lane or passage, the vertical tube portions are disposed in an array of parallel rows and parallel columns, with the rows and columns extending perpendicular to each other. The tubes may be arranged in a so-called "square pitch" array or in a so-called "triangular pitch " array. In a square pitch array the spacing between adjacent columns is the same as the spacing between adjacent rows, and each column contains a tube in every row which it intersects, and vice versa. In a triangular pitch array, the spacing between adjacent columns is different from the spacing between adjacent rows, and the array is staggered so that each column contains a tube in every other row which it intersects, and vice versa. In the triangular pitch array, the free space between adjacent columns is substantially less than in the square pitch array.
A tube sheet supports the vertical portions of the tubes at their lower ends. The vertical tube portions on one side of the center tube lane are connected to a primary fluid inlet plenum and those on the other side of the center tube lane are connected to a primary fluid outlet plenum. The primary fluid, having been heated by circulation through the reactor core, enters the steam generator through the primary fluid inlet plenum, is transmitted through the tube bundle and out the primary fluid outlet plenum. At the same time, a secondary fluid or feedwater is circulated around the tubes above the tube sheet in heat transfer relationship with the outside of the tubes, so that a portion of the feedwater is converted to steam which is then circulated through standard electrical generating equipment.
Sludge, mainly in the form of iron oxides and copper compounds along with traces of other metals, settles out of the feedwater onto the tube sheet. The sludge deposits provide a site for concentration of phosphate solution or other corrosive agents at the tube walls that results in tube thinning. Accordingly, the sludge must be periodically removed.
One known method for removal of the sludge is referred to as the sludge lance-suction method. Sludge lancing consists of using high pressure water to break up and slurry the sludge in conjunction with suction and filtration equipment that remove the water-sludge mixture for disposal or recirculation. A high velocity water lance is introduced through a handhole and moved along the center tube lane adjacent to the tube sheet. The lance emits a high-velocity water jet perpendicular to the movement of the lance, i.e., parallel to the columns of tubes. The lance is stopped in predetermined positions so that the water jet is directed along the channel or space between adjacent tube columns.
It is essential that the lance nozzle be accurately positioned so that the water jet is confined to the channels between adjacent tube columns and does not directly strike the tubes, particularly the tubes in the row closest to the center tube lane. Correct alignment is particularly important in narrow gap steam generators where triangular pitch tube arrays are used, and where a displacement of as little as 0.01 inch could render the sludge lance ineffective in removing sludge.
One method and apparatus for aligning workpieces such as sludge lance nozzles is disclosed in the copending application of Clark B. Candee, Ser. No. 576,678, filed Feb. 3, 1984, , assigned to the Assignee of the present invention, now abandoned. That method utilizes an eddy current probe mounted on the sludge lance nozzle for producing a maximum output signal when the probe is directly opposite one of the tubes. Other current methods utilize mechanical sensing arms. Neither method can be used while the sludge lance is operating due to mechanical restrictions. Also, the eddy current device tends to be affected by the presence of water mist, vibration, etc. which are present when the lance is operating. In addition, the eddy current device requires delicate adjustment for an operator to catch the peak signal, since the tube returns a reflected signal which is relatively flat near its peak.